This invention relates to porous metal bodies, termed "reticulates", of titanium (Ti) and titanium hydride. By a "reticulate" we refer specifically to an open cell, or open pore (openly porous) metal or metal-like structure of arbitrary size in which a multiplicity of voids, whether cells, pores and/or passages are in open fluid communication with each other.
A recticulate of this invention may be formed with a predetermined relatively uniform pore size and porosity in each unit volume of its structure. A reticulate may also be formed, if so desired, with a wide range of pore sizes in each unit volume. Preferred reticulates of this invention have an "open cell foam morphology" characterized by large pores (or "cells") bounded by strands (also referred to as filaments or ribbons), the pores being interconnected. The strands are generally not circular but irregular in cross-section having an average thickness (measured in a direction at right angle to the direction in which width of a strand is measured) or an average equivalent diameter (assuming a circular cross-section) of less than 10 mils, and preferably in the range from about 0.2 mil to about 5 mils (0.005 inch), hence the term "thin-strand reticulate".
There is very little reference in the prior art to the formation of thin coatings of Ti metal, mostly because from a practical point of view, it can be neither electroplated nor deposited by conventional electroless deposition techniques. We know of no reference to the formation of an open cell foam of Ti metal, or an open cell foam of titanium hydride.
Reticulates of this invention are formed from titanium hydride ("TiH.sub.2 recticulate"), and, from Ti metal derived by sintering the TiH.sub.2 reticulate to obtain the Ti metal reticulate ("Ti reticulate"). A TiH.sub.2 reticulate supported on a refractory material such as a ceramic foam is referred to as "metal-like" because of its physical strength. Each reticulate has a porosity in excess of 30%, preferably in excess of 50%, and most preferably in the range from about 75-98%. Reticulates having a porosity in the range from about 50-80% are sometimes referred to as "felt-like porous bodies"; and, those having a porosity in the upper range are referred to as "sponge metals". See Characteristics and Applications of Sponge Metal" by Eiji Kamijo and Masaaki Honda, in Chemical Economy & Engineering Review, published by Chemical Economy Research Institute (Japan), Dec. 1975, the disclosure of which is incorporated by reference thereto as if fully incorporated herein. It is essential that the reticulates of this invention be essentially free of titanium carbide (TiC), that is, there should be less than 0.5 percent by weight (% by wt) of TiC, and preferably no more than 0.1% by wt more TiC than contained in the starting (original) TiH.sub.2 powder.
Our reticulates, like the Kamijo et al sponge metals have a framework extending in all directions in a continuous reticulated structure, all the pores being connected, though their pores are said to have a spherical shape. Such sponge metals cannot be produced by conventional `loose sintering` or `press sintering` of powder metals. These Kamijo et al sponge metals include those of pure metals such as nickel, copper and iron, as well as alloys such as Nichrome, though there is no enabling disclosure as to how such sponge metals may be prepared. Neither is there any suggestion that metal hydrides of any kind may be used to produce the sponge metals.
Hydrides of titanium are unique in that they are non-stoichiometric compounds thought to comprise interstitially held hydrogen in varying amounts. Titanium hydride is generally represented as TiH.sub.2 and will be so represented hereinafter.
The Ti (metal) reticulate of this invention is in its most preferred embodiment, a "metal sponge" or "sponge metal" which should not be confused with "metal foam". Metal foam consists of gas-containing discrete cells distributed in a metal matrix in a generally uniform manner, each cell being entirely enclosed and generally being not connected to any neighboring cell. Similarly, the TiH.sub.2 reticulate is an open cell structure.
Numerous methods have been employed in the production of porous metal bodies, particularly sponge metals, in the past decade or so, because of the enhanced interest in utilizing such bodies in specialized applications identified in Kamijo et al, supra. One of such applications is the production of sponge metal sheets used for sound absorption, insulation against heat and cold, and as demisters, inter alia. These methods include (a) sintering of metal particles, (b) the use of materials which liberate gas at elevated temperatures to cause voids in molten metal, (c) the use of slip casting techniques in which metal particles are suspended in a variety of liquid or solid binders and then heated to eliminate the solvent or binder, and (d) electroless coating or electroplating of porous materials.
How porous metal bodies are derived from powder metals is extensively discussed in texts and articles on powder metallurgy, and it is well known that such powder metal-derived porous bodies have relatively low porosity, less than about 30%, and that their pore size is determined by the size of the powder metal particles. What is not so well known is that porous bodies derived from powder metals have characteristics which are quite different from those of sponge metals.
Further, a powder of Ti metal is generally regarded as being difficultly sinterable and we know of no teaching in the prior art that TiH.sub.2 powder may be substituted for Ti powder; or, that there was any compelling reason for forming a TiH.sub.2 reticulate; or, that such a TiH.sub.2 reticulate may, if desired, be converted by sintering, to a Ti reticulate.
With respect to forming a sponge metal by the evolution of gas in a molten metal, it is acknowledged to be an unsatisfactory way of producing a reticulate because neither the porosity nor the pore size can be controlled within a preselected range; not only from one batch to another, but within the same batch.
Reducing to practice the concept of leaching solids to form a porous metal structure is arduous yet simple, but the effectiveness of such a procedure is very much related to the properties of the metal, and also, the solid which is to be leached from the metal, with the result that this method is now consistently disfavored. For example, U.S. Pat. No. 3,218,684 teaches that a cast tubular magnesium reticulate is formed by pouring molten magnesium over prilled NaCl pellets in a mold. Moreover, this leaching process does not produce "thin-strand" reticulates.
We do not know of any method for the electroless coating of synthetic resins, such as polyurethane foam, with metals such as nickel, copper, etc. which method is also applicable to titanium; and we know that titanium can be electroplated in molten salt but this method would be inapplicable to coating synthetic resinous foams. Producing porous Ti reticulates by sintering TiH.sub.2 initially appeared unpromising because of the well-known difficulty of removing the internal hydrogen sufficiently completely so as not to leave an embrittled Ti structure. See "Effect of Hydrogen on Titanium and its Alloys" by Paton, N. E. and Williams, J. C., Hydrogen in Metals edited by Bernstein, I. M. et al., American Society for Metals, (1974).
All the prior art methods are subject to numerous drawbacks among which are (i) the reticulate's porosity is non-uniform and generally less than 50%, (ii) its pore size is not controllable within a desirably narrow range, and (iii) the methods do not lend themselves to the manufacture of relatively large shaped reticulates, for example, parallelepipeds up to 5 ft.times.5 ft.times.6 in.
The aforesaid drawbacks are said to be overcome in a method disclosed in U.S. Pat. No. 3,111,396 comprising coating an open cell polymethane foam with a suspension of a powdered metal or metal oxide in a fluid, slowly drying the impregnated organic structure, heating the impregnated organic structure to decompose the organic structure and the fluid while closely retaining the shape and size of the original organic structure, and then heating the impregnated carbon-powdered material structure to further join the powder into a continuous form. A slurry was formed with finely divided metal, or metal oxide, or other metal compound in a fluid, optionally with a decomposable thickening agent, a metal hydride or a salt which will perform or provide for some binding action. An organic cellular or porous structure was coated with the slurry, and after drying, heated to a (first) temperature sufficient to reduce the organic structure to carbon, though there is no teaching as to what critical atmospheric control accomplishes this. This carbon structure, coated with powder of the original slurry is then heated to a (second) higher temperature than before to assure full carbonization of the organic structure. The fully carbonized structure which is essential to maintain the coherency of the particles to be sintered, is then heated to a (third) still higher temperature to sinter the powder into a foam product.
The drawback of the aforesaid process is that sintering the fully carbonized structure results in the formation of a substantial quantity of carbides of those metals which are reactive with carbon at sintering temperatures even in an atmosphere which is inert with respect to Ti. Ti is such a reactive metal.
To produce the desired Ti metal reticulate which is essentially free of metal carbide, it is critical that essentially all carbon and carbon-containing compounds ("carbonaceous material") be removed prior to commencing sintering of a sinterable powder. As far as we have determined, only the hydrides of Ti lend themselves to this application because it is a first peculiarity of TiH.sub.2 that it decomposes at a temperature much lower than the decomposition temperature of the oxides.
A second peculiarity of TiH.sub.2 is critical to the formation of the sintered metal reticulates of this invention, namely, that upon sintering, the hydride undergoes a shrinkage in volume. For example, TiH.sub.2 undergoes at least 10%, and generally about a 15% reduction in volume, so that there is a substantial contraction in volume from the original volume of the organic porous material impregnated with slurry. This shrinkage of volume of metal hydride particles exerts a particle-to-particle pressure sufficient to form a diffusion bond and sinter the particles. Such a diffusion bond was known to be formed only under relatively high pressure, as for example taught in "Titanium Powder Metallurgy by Decomposition Sintering of the Hydride" by Greenspan, J. et al. Titanium Science and Technology edited by Jaffee, R. I. et al., Vol. 1, Plenum Press (1973).
A third peculiarity, critical to our invention, is that a slurry of TiH.sub.2 powder and a fugitive binder on a fugitive pore-former produces a self-supporting binderless TiH.sub.2 reticulate when the binder and pore-former are driven off by heating in an inert oxygen-free atmosphere; or, if the slurry is coated on an inorganic reticulate pore-former, it produces a pore-former-supported binderless TiH.sub.2 reticulate. The latter is independently useful as a hydrogenation catalyst for the hydrogenation of vegetable oils and the like. TiH.sub.2 is a known hydrogenation catalyst (see "Supported Titanium Hydride as a Hydrogenation Catalyst" by Lisichkin, G. V., et al, Vses Khim. Oeva 1978, 23(23) 356-7, Russia). Because this binderless structure can be freed of carbonaceous material at a temperature below about 400.degree. C. at which TiH.sub.2 starts to decompose, it becomes possible to sinter the TiH.sub.2 reticulates under helium or argon at essentially atmospheric pressure, to produce the thin-strand Ti reticulates of this invention.
Metal hydrides and salts of metals have been used in the prior art as binders, particularly the metal hydrides, to produce metal foam as a result of their decomposition (see U.S. Pat. No. 3,794,481). As will be evident, the TiH.sub.2 used in the process of our invention is not a binder, and its decomposition upon sintering produces no foaming. Further, there was no reason to expect that a slurry of decomposable TiH.sub.2 might be sintered without reaction with the components of the slurry at the elevated temperatures at which decomposition of TiH.sub.2 occurs.
The desirability of fabricating a Ti reticulate for service as an anode presented itself because of unremitting efforts to solve a problem endemic to conventional chloralkali electrolytic cells. In such cells, any restricted circulation of electrolyte through an expanded metal ("mesh") or porous metal anode contributes to a deleterious bubble overpotential so termed because of Cl.sub.2 bubbles clinging to the anode, thereby reducing the active surface area and increasing the electrical resistance. Anodes of Ti mesh coated with a Pt group metal oxide are favored in industrial chloralkali electrolytic cells because it has been found that the less restricted the circulation of electrolyte, the lower the electrode overpotential.
Recognizing however, that a practical reticulate anode is preferably a relatively thick parallelepiped--industrial anodes range from about 5 cm to about 20 cm thick, and may be from 1 ft wide.times.1 ft long, to as much as 5 ft wide.times.6 ft long,--it was far from evident how effectively the proclivity for bubble formation could be countered by improved circulation attributable to large pores and high porosity of the anode.